The present invention relates to the field of X-ray diffraction and, more particularly, to a system for local X-ray excitation by monochromatic X-rays.
Prior chemical analysis techniques include electron probe microanalysis and X-ray fluorescence analysis. In both, X-rays produced by bombardment of a sample are analyzed to determine sample composition. The electron probe can be highly localized on the sample, but produces an undesirably high background noise level. This is primarily due to the presence of bremsstrahlung, along with characteristic lines of the sample. In some cases, the background is equal in magnitude to an output signal representing a sample concentration as high as 0.2% to 1.0%. An electron probe can also destroy radiation sensitive materials, and is not practical for use in analyzing uncoated insulators because of a charging effect.
In chemical analysis with an X-ray probe, collimated X-rays are often obtained by selecting a small segment of a beam. However, this is wasteful of energy, requires a high power X-ray source and does not provide small excitation regions.
The prior systems known to applicant for improved detection limits in X-ray fluorescence analysis rely on polarized radiation or secondary fluorescence radiation. In the former case, X-rays are scattered from a polarizing target and then directed onto a sample to cause fluorescence. Secondary fluorescence systems irradiate samples with independenty produced fluorescence X-rays. However, neither system provides adequate localization of X-rays for microanalysis, i.e., analysis of regions on the order of 100 microns or less in diameter.
A number of prior X-ray monochrometers use diffraction crystals bent in more than one direction. Such doubly curved crystals roughly monochromatize and approximately focus X-rays, as described in the following U.S. patents: Hammond et al, U.S. Pat. No. 3,772,522; Hammond et al, U.S. Pat. No. 3,777,156; Berreman, U.S. Pat. No. 2,853,617; Carroll, Jr., U.S. Pat. No. 4,203,034; and Furnas, U.S. Pat. No. 3,898,455. The crystals do not accurately focus or monochromatize an X-ray beam because they conform to Johann geometry rather than the more accurate Johannson configuration, or are bent in spherical or cylindrical shapes. The Hammond et al '522 patent briefly discusses a crystal geometry which would permit point focusing, but discards it as being too difficult to make. The point focusing geometry is described by Hammond as having the equivalent of Johannson geometry in the plane of a Rowland circle, and being toroidally bent in a perpendicular direction to a radius equal to D.sub.R sin .sup.2 .theta., where D.sub.R is the diameter of the Rowland circle and .theta. is the Bragg angle. The Furnas patent also describes a toroidal crystal arrangement, but the crystals are used in the Johann geometry and therefore provide poor monochromatization and poor focusing.
Certain monochrometers and spectrometers have multiple diffraction crystals selectable for use with X-rays of different wavelengths. For example, wavelength dispersive spectrometers (WDS) typically contain multiple diffractors rotatable about an axis for switching between wavelengths. However, such diffractors are typically arranged with the concave surfaces of the crystals facing outwardly from the axis of movement. In these cases the diffractors and image points are also moved relative to the X-ray source and to each other throughout the analysis process. Movement is required when switching from one diffractor to another, and for scanning the wavelength range of the diffractor being used.
The Furnas patent, in the embodiment of FIG. 7 thereof, discloses a toroidal diffraction element made up of three sections focusing X-rays between the same source and image points. However, the sections are used simultaneously and are not selectable to vary the wavelength of focused X-rays. Even if the X-ray paths to two of the sections were blocked for the purpose of selecting a wavelength diffracted by the other, X-rays would impinge upon the target from significantly different directions depending upon the section selected. This difference in direction makes the Furnas device impractical for microanalysis because X-rays of the different wavelengths would encounter imperfections in a sample surface from different directions, causing the results to vary from one wavelength to another.
Therefore, it is desirable in many applications to provide an improved system for X-ray microanalysis which does not require a high power X-ray source and is able to provide a high signal to background ratio from very small regions of a sample.